According to Markus Neuhauser and Sven Krackow, from the Institute of Medical Informatics, Biometry and Epidemiology at University Hospital Essen, in Germany, the risk of a child being born with Down Syndrome is also dependent on how many existing siblings the child has and how big the gap is between the child and his immediate preceding sibling.

Neuhauser and Krackow reviewed and analysed data from 1953 and 1972. They found that other factors, besides the mother’s increasing age, were linked to the number of Down Syndrome cases. Down Syndrome rates were significantly higher in older mothers in their first pregnancy than in older mothers who had already had children. Only late first pregnancies were more likely to produce a Down Syndrome baby, not late second or third pregnancies. In addition, the larger the gap between pregnancies, the higher the rates of Down Syndrome.

Neuhauser and Krackow’s paper provides evidence that older mothers, who give birth to children with Down Syndrome, have a relaxed stringency of quality control of embryos, which increases the probability that these women will bring children with developmental defects to full term. They believe that this relaxed filtering stringency is an adaptive maternal response and it might explain why the rate of Down Syndrome accelerates with increasing maternal age.

These findings have important implications for the prevention of abnormal foetal development, the authors say. They conclude that “clearly, identification of the relaxation control mechanisms and therapeutic restoration of a stringent screen holds promise not only for Down Syndrome.”